Metabolic substrate utilization can be an important factor in the pathophysiology of certain disorders. In particular, defects in metabolic substrate utilization have been observed in individuals that are suffering from certain metabolic disorders, such as obesity and diabetes. For example, obese individuals and diabetic individuals can have reduced basal fat oxidation rates and reduced postprandial carbohydrate oxidation rates compared to control individuals. Defects in metabolic substrate utilization have also been observed in individuals that are at risk of developing certain metabolic disorders. For example, prediabetic individuals can have similar defects in fat oxidation as diabetic individuals, and this observation has been used to associate defects in fat oxidation with the progression of diabetes. In addition, defects in metabolic substrate utilization have been observed in individuals that are recovering from certain metabolic disorders. For example, previously obese individuals can have lower fat oxidation rates compared to control individuals. Also, these previously obese individuals sometimes do not increase fat oxidation as quickly in response to additional fat intake compared to control individuals.
Metabolic substrate utilization is typically assessed by measuring the respiratory quotient. Although the respiratory quotient can be useful for determining relative rates of carbohydrate oxidation and fat oxidation, the respiratory quotient typically does not provide information regarding the underlying causes of differences in carbohydrate oxidation and fat oxidation. In particular, it is often unclear whether differences in carbohydrate oxidation and fat oxidation are due to differences in availability of carbohydrates and fats or due to a predisposition towards carbohydrate oxidation or fat oxidation.
It is against this background that a need arose to develop the apparatus and methods described herein.